Conventionally, techniques of compensating for nonlinear distortion of a power amplifier used in a base station or the like have been proposed in the field of radio mobile communication. Among such technique, there is a technique of compensating for a nonlinear distortion using a plurality of distortion compensation units and dual band digital pre-distortion (DPD) when signals at multiple frequency bands are concurrently amplified with a common power amplifier. Further, there is a power amplifier using a dual band DPD that is provided with a Volterra filter of a dual-input truncated Volterra model in front of a two-dimensional look up table (LUT) and also compensates for a second-order distortion. In addition, there are a technique of performing distortion compensation by adding harmonics and intermodulation distortions generated in other bands to signals having inverse characteristics, and a technique of performing distortion compensation by solving a nonlinear simultaneous equation.
[Patent Literature 1] Japanese Laid-open Patent Publication No. 2002-084146
[Patent Literature 2] Japanese Laid-open Patent Publication No. 2012-227881
[Patent Literature 3] Japanese Laid-open Patent Publication No. 2014-003527
[Non-Patent Literature 1] Bassam, S. A., Helaoui, M., Ghannouchi, F. M., “2-D Digital Predistortion (2-D-DPD) Architecture for Concurrent Dual-Band Transmitters,” IEEE Transactions on Microwave Theory and Techniques, vol. 59, no. 10, pp. 2547-2553, October 2011.
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[Non-Patent Literature 4] Yuelin Ma, Yasushi Yamao, “Spectra-Folding Feedback Architecture for Concurrent Dual-Band Power Amplifier Predistortion,” IEEE Transactions on Microwave Theory and Techniques, vol. 63, no. 10, pp. 3164-3174, October 2015.
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However, it is sometimes difficult to obtain sufficient distortion compensation performance in a distortion compensation apparatus, which may cause deterioration of radio quality. For example, it is assumed a case where signals at multiple frequency bands are concurrently amplified by a common power amplifier. In this case, under a condition that center frequencies of two bands have a relationship of approximately twice (for example, 4.6 GHz and 9 GHz), a distortion of an even order (hereinafter, simply referred to as an “even-order distortion”) may occur at a frequency corresponding to a difference between these center frequencies or at a frequency corresponding to twice the center frequency which is lower than the other. In addition, there is a case where the even-order distortion occurs similarly in a case where isolation of a combiner is insufficient even when signals at multiple frequency bands are concurrently amplified by a plurality of power amplifiers and combined. If a distortion compensation apparatus is configured so as to compensate for such an even-order distortion, a circuit scale of the distortion compensation apparatus increases in some cases.